Transport & Nanostructures Group

Our first publication of 2022 “Thermal conductivity of Thue–Morse and double-period quasiperiodic graphene-hBN superlattices”  has just been published in the International Journal of Heat and Mass Transfer.

Nanostructured superlattices are promising materials for novel electronic devices due to their adjustable physical properties. Periodic superlattices facilitate coherent phonon thermal transport due to constructive wave interference at the boundaries between the materials. However, it is possible to induce a crossover from coherent to incoherent transport regimes by adjusting the superlattice period. In 2018 we observed such crossover in periodic graphene-boron nitride nanoribbons as the length of individual domains was increased. In general, transport properties are dominated by translational symmetry and the presence of unconventional symmetries leads to unusual transport characteristics. In 2020 we showed that the quasiperiodicity can suppress coherent phonon thermal transport in superlattices following the Fibonacci quasiperiodic sequence, which lie between periodic and disordered structures. Finally, in our latest publication we perform non-equilibrium molecular dynamics simulations to investigate phonon heat transport in graphene-hBN superlattices following the Thue-Morse and double-period quasiperiodic sequences, and show that coherent transport is indeed suppressed due to phonon localization caused by increase in superlattice period. We also obtain a general expression for conductivity as a function of quasiperiodic generation and supercell size, which might be useful for superlattice following other sequences.

This is the latest publication, but hopefully not the last, from Isaac’s PhD dissertation. It was completely carried out within our research group at UFRN and UFPE, and we are grateful for the computational support provided by the supercomputing center at UFRN (NPAD).

Encontro de Física do Norte e Nordeste (North and northeast physics meeting) promoted by the Brazilian Physical Society is the second largest Physics meeting in Brazil, gathering more than 500 researchers from the north and northeast regions of Brazil. In 2021, the event takes place online, hosted by Universidade Federal de Pernambuco, from 18 to 20 of October.

I contributed to the local organization of the event, but also presented a talk entitled “Electronic structure, elastic properties and thermal conductivity of pentadiamond: First-principles calculations and machine-trained potentials“, in which I present some of our results recently published in Carbon Trends.

My first single-author publication, and third publication of 2021 “Investigating mechanical properties and thermal conductivity of 2D carbon-based materials by computational experiments”  has just appeared in the 2020 Rising Stars Special Issue of Computational Materials Science.

It is a short review submitted upon invitation by the editors of Computational Material Science. In this publication I cover some of our results concerning the mechanical properties and the lattice thermal conductivity of two-dimensional carbon-based materials, obtained by state-of-the-art computational experiments. I also explore the direct relationship between mechanical strength and lattice thermal conductivity in 2D materials, and analyze the outliers. I had the idea of writing this review for a few years, and the 2020 Rising Stars Special Issue was the perfect opportunity to finally execute the idea.

The results presented in the review were obtained in collaboration with several colleagues from whom I learned so much over the years, in particular Davide Donadio, Bohayra Mortazavi, Zheyong Fan and Ari Harju. The research was funded by the Max Planck Society, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The computational resources were provided by Rechenzentrum Garching of the Max Planck Society, the Jülich Supercomputing Centre, and the High Performance Computing Center (NPAD) at Universidade Federal do Rio Grande do Norte.

Our second publication of 2021 “First-principles investigation of electronic, optical, mechanical and heat transport properties of pentadiamond: A comparison with diamond”  has just appeared in Carbon Trends

Pentadiamond is a carbon allotrope consisting of hybrid sp2 and sp3 atoms, which has recently been predicted to be stable and synthesizable. We employed first-principles calculations to explore the electronic structure, optical characteristics, mechanical response and lattice thermal conductivity of pentadiamond, performing a direct comparison with the corresponding properties in diamond. Density functional theory calculations with the HSE06 functional predicts indirect electronic band gaps of 3.58 eV and 5.27 eV for pentadiamond and diamond, respectively. Pentadiamond shows large absorption in the middle UV region, where diamond does not absorb light, consistent with its smaller band gap. The elastic modulus and tensile strength of pentadiamond are 496 GPa and 60 GPa, respectively, considerably lower than the corresponding values for diamond. Finally, the lattice thermal conductivity was examined by solving the Boltzmann transport equation with anharmonic force constants evaluated via state-of-the-art machine-learning interatomic potentials. We predict a thermal conductivity of 427 W/m-K for pentadiamond, less than one fifth of the corresponding quantity for diamond. Our results provide a useful vision of the intrinsic properties of pentadiamond, but also highlight some of its disadvantages in mechanical strength and heat conduction when compared to diamond.

This work results from collaboration with colleagues at Leibniz Universität Hannover and the Persian Gulf University.

Our latest publication “Majority-vote model with limited visibility: An investigation into filter bubbles” is a little outside our main line of work. Nonetheless, any physics problem is interesting, and opinion formation models are a lot of fun to investigate. Indeed, the dynamics of opinion formation in a society is a complex phenomenon where many variables play essential roles. Recently, the influence of algorithms to filter which content is fed to social networks users has come under scrutiny. Supposedly, the algorithms promote marketing strategies, but can also facilitate the formation of filters bubbles in which a user is most likely exposed to opinions that conform to their own.

In the two-state majority-vote model, an individual adopts an opinion contrary to the majority of its neighbors with probability , defined as the noise parameter. Here, we introduce a visibility parameter  in the dynamics of the majority-vote model, which equals the probability of an individual ignoring the opinion of each one of its neighbors. For  each individual will, on average, ignore the opinion of half of its neighboring nodes. We employ Monte Carlo simulations to calculate the critical noise parameter as a function of the visibility  and obtain the phase diagram of the model. We find that the critical noise is an increasing function of the visibility parameter, such that a lower value of  favors dissensus. Via finite-size scaling analysis we obtain the critical exponents of the model, which are visibility-independent, and show that the model belongs to the Ising universality class. We compare our results to the case of a network submitted to a static site dilution and find that the limited visibility model is a more subtle way of inducing opinion polarization in a social network.

This work is a collaboration with my colleagues André Vilela at Universidade de Pernambuco, Laercio Dias and Luciano Rodrigues at Universidade Federal do Rio Grande do Norte, and H. E. Stanley at Boston University.